I've become rather a fan of portable applications recently. Portable application are great because they don't need to be installed and uninstalled. They can be run from a pen drive or a folder on your hard drive. If you decide you don't need them you can just delete them. Installed application can sometimes leave a lot of clutter on your machine by peppering your hard drive with files and your registry with settings that are not removed.
These are application that you can store and run from a pen drive. To be portable an application must be written so that it does not use the windows registry or the GAC. The application has to be self contained, with an executable and all required DLLs held locally under a single folder.
You will find a good list of FREE Portable Applications at http://www.portablefreeware.com/